The present invention relates to a method for thermal monitoring of a converter for an electric motor of a vehicle, wherein the temperature of the converter is measured. The present invention furthermore relates to a monitoring device for thermal monitoring of the converter, having a measuring facility for measuring the temperature of the converter.
Internal combustion engines in motor vehicles are usually cooled by a liquid cooling system. If this cooling system fails, then the temperature of the combustion engine rises as a function of the load figure and ambient conditions. This rise can be observed by means of a corresponding display element in the field of vision of the driver. When a critical threshold is exceeded an additional acoustic and/or visual signal is for the most part also generated which makes the driver aware of this status. The so-called time constant (time in which a certain temperature difference is overcome) for the temperature rise lies in the range of several minutes and enables the vehicle driver to still safely stop the vehicle after the malfunction is detected.
In the case of converters for the electric drive of motor vehicles a liquid cooling system having a similar structure is likewise frequently used. Since the mass of a converter is however considerably less than that of a combustion motor, the thermal time constants here lie only in the range of several tens of seconds, in other words the period of time between the detection of an excessive temperature rise and the possible destruction of the converter as a result of excess temperature is relatively short.
In conventional industrial applications a warning is issued after the excess temperature is detected. In the event of a further temperature rise a shutdown occurs in order to protect the converter. When the cooling is functioning normally this can be done without any problem. However, if the cooling fails as a result of a leak in the system, then the temperature rises so quickly under load on account of the small thermal time constants that there are only a few seconds between temperature warning and shutdown. In certain cases this is possibly insufficient in order to bring the vehicle safely to a standstill. For example, during a journey on a highway the converter is operating only under a moderate load. In other words, even in the event of failure of the cooling system the temperature of the converter certainly does not reach the warning threshold. During an overtaking maneuver, however, the load is increased from that state up to maximum power. If the cooling system has failed, the temperature rises very rapidly above the threshold for warning and shutdown, which means that the available power may be reduced or fall away completely midway through an overtaking maneuver. This can result in critical driving situations.
A method and an apparatus for detecting a contamination level of an operational converter device are known from the publication DE 10 2004 018 578 A1. The apparatus comprises a temperature model, a temperature sensor and an evaluation circuit. The temperature sensor is positioned on the component of the converter device whose temperature is to be measured. The temperature model in question is an already known temperature model for a cooling element. This temperature model is used to determine an expected cooling element temperature, depending on an actual power loss and an actual coolant temperature. If the cooling element of the converter device is contaminated, a measured cooling temperature is higher than the estimated cooling element temperature of the temperature model. A negative value is obtained as a comparison value. This signals that the cooling element of the power section of the converter device is operating worse than intended.
The object of the present invention thus consists in being able to better monitor a converter for an electric motor of a vehicle.